Common mode noise reduction circuit, differential signal transmitting apparatus, differential signal transmitting system and car electronics device

Abstract

A common mode noise reduction circuit works with a transmission signal output circuit that has a first and a second output terminals and transmits differential signals from the first and second output terminals. The common mode noise reduction circuit includes: a first generating circuit to generate electric current to input to or receive electric current from the first output terminal; a second generating circuit to generate electric current to input to or output receive electric current from the second output terminal; and a control circuit to control the first and second generating circuits so that in synchronism with a drive control clock of the transmission signal output circuit, the first and second generating circuits generate current pulses to reduce common mode noise of the differential signals to be transmitted.

What is claimed is: 
     
       1. A common mode noise reduction circuit for a transmission-signal output circuit having a first and a second output terminals configured to transmit differential signals from the first and second output terminals, the common mode noise reduction circuit comprising:
 a first generating circuit configured to generate electric current to input to the first output terminal or receive electric current from the first output terminal; 
 a second generating circuit configured to generate electric current to input to the second output terminal or receive electric current from the second output terminal; and 
 a control circuit configured to control the first and second generating circuits so that in synchronism with a drive control clock of the transmission signal output circuit, the first and second generating circuits generate current pulses to reduce common mode noise of the differential signals to be transmitted, 
 wherein the transmission signal output circuit comprises: 
 a first current source having a first end connected to a direct current source; 
 a second current source having a first end connected to the ground; 
 a first switch connected between the first output terminal and a second end of the first current source; 
 a second switch connected between the second output terminal and the second end of the first current source: 
 a third switch connected between the first output terminal and a second end of the second current source; 
 a fourth switch connected between the second output terminal and the second end of the second current source; and 
 a terminal resistor connected between the first output terminal and the second output terminal, and 
 the transmission signal output circuit is configured to, in synchronism with the drive control clock, generate drive control signals to the first, second, third and fourth switches to power on and off at least one of the first, second, third and fourth switches to transmit the differential signals from the first and second output terminals. 
 
     
     
       2. The common mode noise reduction circuit of  claim 1 , wherein
 the control circuit comprises a memory apparatus configured to store a control table including a plurality of cases representing combinations of transition conditions of the first, second, third and fourth switches and control signals that control the first and second generating circuits according to transition of the differential signals to be transmitted from the first and second output terminals, and 
 the control circuit is configured to, according to signal voltages of the differential signals transmitted from the first and second output terminals, search the cases for a case in which the common mode noise becomes a minimum level with a timing and a signal width of the control signal being changed, and also search the timing and the signal width, and then set the search result to control the first and second generating circuits. 
 
     
     
       3. The common mode noise reduction circuit of  claim 2 , wherein the control circuit is configured to search the case, timing and signal width which minimize the common mode noise while the transmission signal output circuit is in operation, and adaptively control the first and second generating circuits according to the search result. 
     
     
       4. The common mode noise reduction circuit of  claim 1 , wherein each of the first and second current sources is configured by a transistor, and each of the first, second, third and fourth switches is configured by a transistor. 
     
     
       5. The common mode noise reduction circuit of  claim 1 , wherein each of the first and second generating circuits comprises:
 a third output terminal connected to the first or second output terminal; 
 a third current source having a first end connected to the direct current source; 
 a fifth switch connected between a second end of the third current source and the third output terminal; 
 a fourth current source having a first end connected to the ground; and 
 a sixth switch connected between a second end of the fourth current source and the third output terminal, wherein 
 the control circuit is configured to generate a switch control signal to control the fifth and sixth switches. 
 
     
     
       6. The common mode noise reduction circuit of  claim 1 , wherein each of the first and second generating circuits comprises:
 a third output terminal connected to the first or second output terminal; 
 a series circuit configured by a fifth switch and a third current source, and connected between the direct current source and the third output terminal; 
 a series circuit configured by a sixth switch and a fourth current source, and connected between the ground and the third output terminal, wherein 
 the control circuit is configured to generate a switch control signal to control the fifth and sixth switches. 
 
     
     
       7. The common mode noise reduction circuit of  claim 5 , wherein each of the third and fourth current sources is configured by a transistor, and each of the fifth and sixth switches is configured by a transistor. 
     
     
       8. The common mode noise reduction circuit of  claim 5 , wherein the third current source and the fifth switch are configured by one transistor, and the fourth current source and the sixth switch are configured by one transistor. 
     
     
       9. The common mode noise reduction circuit of  claim 1 , wherein the control circuit is configured to control the first and second generating circuits to generate electric current to input to the first or second output terminal or receive electric current from the first or second output terminal for a predetermined period at a predetermined timing. 
     
     
       10. The common mode noise reduction circuit of  claim 9 , wherein the predetermined period and predetermined timing are previously set in the control circuit according to a measurement result of the common mode noise of the transmission signal output circuit. 
     
     
       11. A differential signal transmitting apparatus comprising the common mode noise reduction circuit and the transmission signal output circuit of  claim 1 . 
     
     
       12. The differential signal transmitting apparatus of  claim 11 , in which the differential signal transmitting apparatus is configured by a semiconductor integrated circuit. 
     
     
       13. The differential signal transmitting apparatus of  claim 11 , wherein the differential signal transmitting apparatus is configured to transmit the differential signals that are image data for display. 
     
     
       14. A differential signal transmitting system comprising the differential signal transmitting apparatus of  claim 11  and a differential signal receiving apparatus configured to receive the differential signals from the differential signal transmitting apparatus. 
     
     
       15. A differential signal transmitting system that is an image display system, comprising the differential signal transmitting apparatus of  claim 13  and a differential signal receiving apparatus configured to receive the differential signals from the differential signal transmitting apparatus. 
     
     
       16. A car electronics device comprising the differential signal transmitting system of  claim 15 .